This invention relates to lasers and more particularly to unstable resonator optical cavities for a laser such as a gas laser wherein the laser action takes place and from which an output laser beam emerges.
The cavity optical axis of optical resonators, whether they be stable or unstable resonators, is defined as a line perpendicular to the reflecting surface of both the primary and feedback mirrors. In the absence of medium aberrations, this line is a line through the centers of curvature.
A conventional unstable resonator optical cavity in a laser such as a flowing gas laser includes a primary reflection surface at one end of the cavity and a smaller feedback reflection surface at the other end of the cavity. In the usual aligned configuration, both reflection surfaces are centered on the cavity optical axis. This arrangement is such that any ray of radiation along the optical axis, upon repeated reflection between the primary and feedback reflection surfaces will progressively move away from the optical axis of such conventional resonators and eventually clear the outer edge of the feedback mirror and escape from the cavity as output radiation. In other words, the rays initially along the optical axis will "walk out" of the optical cavity and for this reason, the optical cavity is referred to as an "unstable" cavity or resonator.
The output laser beam from the unstable resonator cavity of the sort described above (if the laser beam is close to equiphase across its cross section), exhibits a characteristic Fraunhofer pattern when its far field is examined in cross section. The characteristic Fraunhofer pattern is produced when a light beam of uniform phase front across the beam is either examined far from its source, or brought to a focus. Further, the total radiation flux or power which can be extracted from the laser medium is a function of the total flux within the optical cavity, the small signal gain coefficient g.sub.o, the cavity length, and the optical extraction efficiency .eta..
Another unstable resonator cavity that operates more or less the opposite of that described above is disclosed in U.S. Pat. No. 3,873,942.
Briefly, in this type of cavity the feedback mirror, which may assume various configurations, may be disposed along one or more edges of the cavity (for a rectangular cavity) or be annular in shape for a cylindrical cavity. For this type of unstable cavity, the feedback mirror does not intercept and is spaced from the optical axis and the reflection surfaces are shaped to reflect radiation within the cavity which is initially substantially parallel to the cavity axis at the extreme lateral edges of the reflection surfaces so that radiation progressively moves inwardly toward the optical axis until it clears the inside edge of the feedback reflector and escapes from the cavity as output radiation.
For a discussion of an interesting prior art unstable resonator utilizing a plane presumably rectangular primary reflection surface and a convex cylindrical feedback mirror in combination with an objective output lens wherein one side of the output beam just clears an edge of the feedback mirror and the focus of the objective output lens coincides with the imaginary center of the output beam, reference is made to "Solid Laser With a High Spatial Coherence of Radiation", by Yu. A. Anan'ev, et al., published in the Soviet Journal of Quantum Electronics, Vol. 1, No. 4, January-February, 1972.
It can be shown that the laser output power for a given laser peaks at some coupling factor and tapers off to zero on either side of that peak. If it is preferred to operate at the maximum output power, it is usually strived to design the optical cavity to operate at the peak of the curve, because this results in the highest overall efficiency of the laser. However, the maximum power deliverable to a designated spot size in the far field generally occurs for a different coupling factor.
Many laser applications require that the energy of the laser beam be delivered at a distance from the laser, i.e., there be as much energy as possible in the usable portion of the far field pattern. Further, many of such lasers have a transverse spatially non-uniform gain distribution in the optical cavity and utilize an unstable optical cavity.